The present invention refers to a process and to a roll bending machine for bending plates and iron sheets, according to which the plate to be bent is made to pass between an upper and a lower gripping rolls which are parallely arranged between each other; at least one of these rolls is driven by a suitable drive motor to rotate, and in which at least one of the rolls is upwardly movably supported to exert the required working pressure on a plate to be bent.
The present invention may be advantageously applied to any type of roll bending machine, more simply indicated also as "bending machine", independently from the dimensions of the machine, which may be of two or three roll type depending on the specific use requirements.
Roll bending machine of this type are widely known and described in several prior patents, U.S. Pat. No. 160,647, U.S. Pat. No. 4,312,208 and U.S. Pat. No. 5,218,850 or EP 0 477 751.
The main function of a bending machine is to deform a plate by the pressure exerted by the working rolls, in particular by the upper gripping roll and by the lower gripping roll, which press the same causing an initial bending; a subsequent rotational movement of the rolls draws the plate deforming the same in a cylindrical or partially cylindrical form, for its entire width.
All the plate bending machines presently in use, have a common fundamental drawback relating to the incorrect closing of the edges of the bent plate, caused by the deflection of the rolls; in order to overcome this inconvenience, which moreover is of difficult solution, some extremely complex and expensive solutions have been proposed, in any case it is difficult to meet working requirements.
In fact, the rolls of a bending machine are generally supported and urged at their ends; the strong pressure exerted by the rolls on the plate during the bending phase causes, by reaction, a deflection in opposite directions of the rolls themselves, which is greater in the middle of their length. In particular, the deflection of the rolls, besides by the working pressure, is caused by their slenderness coefficient given by the ratio between the diameter and the axial length of the same roll.
As the two working rolls of a bending machine, because of the reaction exerted by the plate, tend to deflect in opposite directions, spacing each other, this deflection causes a gradual widening of the contact points of the rolls with the plate, the gap of which increases toward the middle area of their length; therefore, as the rolls are more and more spaced from each other when approaching the middle area, this causes a not homogeneous pressure distribution on the iron sheet or plate while it is drawn by the two working rolls of the bending machine.
This changing of the space between the contact points of the rolls with the plate, causes in turn the formation of different bending diameters on the same plate: in practice, in the middle of the plate, because of the larger space between the contact points, greater diameters are obtained with respect to the ends, where the real bending diameters of the plate are close to the nominal bending diameter.
The plate instead of being deformed and bent with a perfectly cylindrical shape, presents a swelling shape, commonly said "barrel" or "cigar" shape, shown for example in FIG. 5; it can be clearly noted that the longitudinal slit between the opposed edges of the bent plate, is more open at the middle than at the ends.
In the attempt of overcoming such an inconvenience, to improve the use of these machines, it is usual for the manufacturers of roll bending machines to shape the rolls with a certain swelling degree or with tapering surfaces suitable to partially compensate their greater deflection at the middle during a bending operation. The upper and the lower rolls of the bending machine present therefore a cylindrical central portion, having a greater diameter, and tapering side parts having shapes and dimensions resulting from the features of the bending machine and the working to be executed; this is shown as example in FIG. 4 of the accompanying drawings.
This swelling shape of the working rolls in a roll-plate bending machine is therefore used to limit the defect of the "barrel" configuration of the iron sheet. However this defect increases when increasing the thickness of the plate; in an attempt to limit the same defect, sometimes it is necessary to reduce the working pressure of the rolls, resulting in a not always satisfactory quality.
The conicity of the gripping rolls causes in turn an opposite defect, which is revealed by a "reel" or saddle-like shape of the iron sheet, shown as example in FIG. 6; it can be noted how the longitudinal slit resulting in this case, is closer in the centre than at the ends.
This defect, which increases when decreasing the thickness of the plate, is particularly critical for thin plates; to attenuate the same, usually it is necessary to proceed in opposite direction to the previous one, by increasing the gripping pressure on the plate between the two working rolls.
Therefore, the manufacturers of bending machines are generally inclined to choose average values of the conicity of the gripping rolls, such to allow a compromise between the "barrel" defect for thick plates, and the "reel" defect for thin plates, consequently acting on the gripping pressure to carry out the necessary corrections.
All that involves therefore a greater constructional complexity for the rolls, a greater complexity in running the bending machine and a consequent limitation in the use of the bending machine itself; in practice, bending machines designed for bending thick plates are not suitable for bending thin plates and vice versa.
In an attempt of overcoming these inconveniences, according to what shown in the prior documents, it has been also proposed to adequately support the working rolls of the bending machine, in their longitudinal direction, by additional thrust rollers, or by a set of small support rollers suitably adjustable in the height. However this solution too gives rise to a great constructional complexity of the machine, high costs, and difficulties in producing tubular elements having a small diameter.